1. Field of the Invention
This invention relates generally to the field of urethane prepolymer formation and, more specifically, this invention relates to a method of forming a urethane prepolymer which exhibits improved capacity for binding aggregate material such as peat and for forming high quality foams.
2. Description of the Art
The use of various polymeric materials to provide support for aggregate material, such as the use of polymeric foam to form a matrix for plant growth-supporting nutrients, for example, is well known.
One form of polymeric matrix which has found wide acceptance in the soil consolidation art is polyurethane foam which comprises a polyether or polyester urethane formed by the process conventionally known as the "one-shot" process, or from a liquid urethane prepolymer which is reacted with an aqueous slurry of aggregate material.
In one form of the slurry reaction referred to above, a hydrophilic liquid urethane prepolymer which comprises an isocyanate-capped polyol with an excess of free isocyanate groups is reacted with a slurry comprising aggregate material, water, and any of various modifying agents such as blowing agents, promoters, accelerators, or other modifying ingredients.
The following U.S. patents, the disclosures of which are hereby incorporated by reference, provide examples of prior approaches to aggregate material consolidation with an urethane foam matrix.
Pruitt et al. U.S. Pat. No. 3,373,009 discloses a method of forming a nutrient plant growth medium from a urethane made by reacting a polyether of polyester compound and a diisocyanate for a period of between about 2 and 4 hours. The prepolymer is then mixed with an aqueous mixture of catalyst, plant nutrient, and selected modifying ingredients to produce a foam urethane polymer intimately incorporating the plant nutrients.
Kistner U.S. Pat. Nos. 3,805,531 and 3,805,532 describe a reaction scheme wherein a urethane prepolymer is formed by mixing a compound having a plurality of active hydrogen atoms, such as polyol, with a stoichiometric excess of an organic polyisocyanate or polyisothiocyanate compound at ambient temperature under essentially anhydrous conditions for a period of several days.
Wood et al. U.S. Pat. Nos. 3,812,619 and 4,241,537 describe a similar reaction conducted for period of about 6 hours.
Such prior methods of forming a urethane prepolymer ultimately used to produce a plant growth medium generally require rather lengthy reaction and/or aging (curing) periods.
One approach to overcoming the problem of the prior art is reflected in Dedolph U.S. Pat. No. 4,329,436 (May 11, 1982) wherein a prepolymer-forming reaction system utilizing reaction temperatures of greater than about 120.degree. C. is disclosed. The inventive reaction of the above-identified Dedolph patent results in relatively viscous prepolymers which exhibit a high degree of cross linking, and which, as a result, upon reacting with water, form open celled, hydrophilic urethane polymers which undergo a minimum of initial expansion and minimal shrinkage following reaction. Such polymers provide enhanced binding ability when used to consolidate aggregate material.